TOPIC EXPLORATION PACK
Theme: Exploration of the
electrolysis of various
molten ionic liquids and
aqueous ionic solution
June 2015
GCSE (9–1) Gateway Chemistry A
Oxford Cambridge and RSA
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GCSE (9–1) Gateway Chemistry A
Topic Exploration Pack
Contents
Introduction ..............................................................................................................................................4
Part 1 .......................................................................................................................................................5
Introduction .......................................................................................................................................5
Activity 1: The electrolysis of distilled water and tap water using a Hoffman Voltameter ....................5
Activity 2: ions in solutions of bottled water.......................................................................................6
Activity 3: The electrolysis of molten zinc chloride and potassium iodide ...........................................7
Part 2 .....................................................................................................................................................10
Introduction .....................................................................................................................................10
Activity: Half equation worksheet .....................................................................................................11
Part 3 .....................................................................................................................................................10
Introduction .....................................................................................................................................10
Activity 1: The electrolysis of sodium chloride ..................................................................................11
Activity 2: Reactivity Series .............................................................................................................12
Part 4 .....................................................................................................................................................13
Introduction .....................................................................................................................................13
This Topic Exploration Pack should accompany the OCR resource ‘Exploration of the electrolysis of
various molten ionic liquids and aqueous ionic solutions’ learner activities, which you can download from
the OCR website.
This activity offers an
opportunity for English
skills development.
Version 1
GCSE (9–1) Gateway Chemistry A
Topic Exploration Pack
Introduction
In the learning of the electrolysis of aluminium, studies have shown that students who are aware of the
‘big picture’ understand and ‘get to grips’ with the smaller concepts easier as they comprehend the
reasons.
Electrolysis is the splitting of compounds by using electricity. It is used in industry to extract and purify
metals as well as non-metals.
The electrolysis of aluminium
This would make an ideal research project or homework in preparation for the taught material.
The following YouTube video as produced by the RSC can be used with the five questions shown below.
1. Why is cryolite added?
To make the material conduct electricity.
2. What are the names of the positive and the negative electrodes?
anode and cathode
3. Which ions move to which electrode?
The positive ions (cations) move to the cathode and the negative ions (anions) move to the anode
4. How long does each carbon electrode last for?
28 days
5. What happens to the remaining carbon when the electrode has worn out?
It is crushed and formed into another electrode.
Key terms: anode (‘+ve’ electrode), cathode (‘-ve’ electrode), cation (‘+ve’ ion) and anion (‘-ve’ ion)
Version 1
GCSE (9–1) Gateway Chemistry A
Topic Exploration Pack
Part 1
Introduction
Objective
To recall that, in electrolysis using inert electrodes, metals (or hydrogen) are formed at the cathode and
non-metals are formed at the anode. It is assumed that students have a full understanding of balancing
equations, ionic bonding and the workings of a simple electrical circuit.
Research
Studies have shown that students have issues understanding that ions dissolve as neutral atoms. This
leads to misconceptions with regards to what carries the charge. They believe that the solution carries
electrons through it. This then leads on to the students forming their own conceptions that the positively
charged ions lose electrons and the negatively charged ions gain electrons from the solution.
Journal of Chemical education 74,7, 1997, 819-823
Activities
Activity 1: The electrolysis of distilled water and tap water using a Hoffman
Voltameter
This activity is designed to get the students to understand that the electrical charge is carried by the ions
dissolved in the solution rather than electrons given by the anode and the cathode to allow electrolysis to
happen.
http://www.nuffieldfoundation.org/practical-physics/electrolysis-water
This experiment should be carried out with distilled water and tap water.
Students will see that the tap water will electrolyse and the distilled water will not.
At this point, students should be given the opportunity to discuss why this might be the case.
The tap water contains dissolved ions which are necessary for electrolysis to take place as they
carry the charge through the solution. Electrons cannot be being transferred into the solution from
the electrodes. The students should then be given commercially available water bottles which will
have the ion content on the label showing that ions are dissolved in the water. This overcomes the
misconception ideas regarding how the current flows and that the two electrodes do not push
electrons into the solution to allow a current to flow.
Version 1
GCSE (9–1) Gateway Chemistry A
Topic Exploration Pack
Activity 2: Ions in solutions of bottled water
The data in Table 1 was obtained from the website:
http://www.cleanairpurewater.com/best_bottled_water.html
With this task, students could be asked to choose which water they would use from the table, and why.
This could be set as a homework task.
Table 1: the pH and mineral content of spring and mineral waters
Arrowhead
Poland
Springs
Fiji
Calistoga
Evian
San
Pellegrino
Trinity
Springs
Volvic
Perrier
pH
6.12-7.6
6.02-7.6
7.5
5.84-6.71
7.18
7.7
9.0
7
5.46
calcium
6-53
3.7-8.2
17
6.0-8.4
78
208
10
147.3
chloride
.73-8.2
1.9-8.8
.74-2.6
2.2
74.3
8
21.5
bicarbonate
42-190
7.2-20
36-44
357
135.5
65
390
fluoride
0.05-1.2
.052-.20
Nd
magnesium
1.5-20
.76-1.4
nitrate
.05-.75
potassium
13
.52
.7
1.3
0.12
1.5-2.9
24
55.9
6
3.4
.13-.75
.06-.57
3.8
.45
1
18
0.7-4.4
.59-.74
1.8-3.8
.75
2.7
6
0.6
sodium
2.1-20
2.4-4.7
3.8-9.5
5
43.6
19.3
9
9
sulfates
.64-32
.81-5.1
Nd 2.1
10
549.2
6
7
33
Teachers could also show students that metals are present in the water by boiling a sample of bottled
water and showing that there is a residue left over at the end.
This can be done by carrying out the experiment from the website below.
http://www.nuffieldfoundation.org/node/1695
The experiment uses seawater but the same principle can be used for bottled water.
Version 1
GCSE (9–1) Gateway Chemistry A
Topic Exploration Pack
Activity 3: The electrolysis of molten zinc chloride and potassium iodide
In the experiments shown in the websites below, the students are aiming to identify the products at each
electrode and show that different ions travel (or are attracted) to different electrodes (anions are
attracted to the anode and cations are attracted to the cathode).
http://www.nuffieldfoundation.org/practical-chemistry/electrolysis-zinc-chloride
http://www.nuffieldfoundation.org/practical-chemistry/reaction-zinc-iodine
Students should now have an understanding of the fact that ions carry the charge in a solution and that
they dissociate in solution (ie are in their ionic state). In these experiments, the products at each
electrode should enable students to understand that the anions and the cations move to opposing
electrodes and are formed there.
The products being produced at each electrode come only from ions that are in solution so students will
get the idea that the positive and negative ions travel to the opposing electrodes. This also includes
hydrogen, as a positively charged ion, as shown with the Hoffman Voltammeter experiment.
Now give out the cards shown and ask the students to match the cards from Set 1 with the
corresponding cards from Set 2.
The numbers in red show the correct answers.
Version 1
GCSE (9–1) Gateway Chemistry A
Topic Exploration Pack
Set 1
cathode
anode
1
2
cation
anion
3
4
ion
5
Version 1
GCSE (9–1) Gateway Chemistry A
Topic Exploration Pack
Set 2
The negatively charged
The positively charged
electrode.
electrode.
1
2
The positively charged
The negatively charged
species that moves to
species that moves to
the cathode.
the anode.
4
3
The charged species
that exists in the molten
state or in solution.
This carries the charge.
5
Version 1
The current is carried
by these.
3, 4 and 5
GCSE (9–1) Gateway Chemistry A
Topic Exploration Pack
Part 2
Introduction
Objective
To predict the products of electrolysis of binary ionic compounds in the molten state.
Research
As has already been discussed, students struggle with the concept that in the molten state the ions are
free to move about. However, it has also been shown that students also struggle with the diagrammatic
representations shown in books, which means they have problems when it comes to understanding half
equations.
Journal Chem Educ 1999, 76, p853
The electrolysis of molten zinc chloride
During the electrolysis of molten zinc chloride, a different reaction happens at each electrode and this
can be represented by separate equations for each electrode. These are called half equations.
Reaction at the cathode
At the cathode (the negative electrode), the positively charged species (in this case, zinc ions, Zn+) move
towards it and collects two electrons (e-) from the electrode. Remember, the electrons come from the
electrode, not the solution.
The half equation for this reaction is:
Zn2+(aq) + 2e-
Zn(s)
Reaction at the anode
At the anode (the positive electrode), the negatively charged species (in this case, chloride ions, Cl-)
move towards it and discharges its electron there.
The half equation (currently incomplete) for this reaction is:
Cl-
Cl + 1e-
However, the overall electrical charge and species must be balanced. Therefore, this half equation must
be multiplied by 2 as two electrons were taken by zinc and a chlorine molecule is Cl2.
Version 1
GCSE (9–1) Gateway Chemistry A
Topic Exploration Pack
Hence the complete half equation for this reaction is:
2Cl-
Cl2 + 2e-
When writing half equations for the anode, it is important that the electron is part of the product and is
therefore on the right hand side of the equation.
At this point we should remember that at the anode, the ions lose electrons so they have been oxidised
and at the cathode, the ions receive electrons so they have been reduced.
So a rule is made: any reaction at the cathode is a reduction reaction and any reaction at the
anode is an oxidation reaction.
Activities
Activity: Half equation worksheet
Complete the following half equations for the electrolysis of the following molten compounds:
1)
NaCl
2Na+(aq) + 2e2Cl2)
2Na(s)
Cl2(g) + 2e-
KBr
2K+(aq) + 2e2Br-
3)
2K(s)
Br2(g) + 2e-
MgCl2
2Mg2+(aq) + 4e4Cl-
4)
2 Mg(s)
2 Cl2(g) + 4e-
Li2O
4Li+(aq) + 4e2O2-
5)
4Li(s)
O2 + 4e-
Al2O3
2Al3+(aq) + 6e3O2-
Version 1
2Al(s)
1.5O2 + 6e-
GCSE (9–1) Gateway Chemistry A
Topic Exploration Pack
Part 3
Introduction
Objective
To describe competing reactions in terms of the different species present in the electrolysis of aqueous
solutions of ionic solutions.
(Consider using CuSO4 and NaCl as practicals.)
Research
It has been shown that when another variable such as water is added (aqueous solutions) then students
become confused as to what is happening in the solution and what is happening within the electrolysis cell.
Chem Educ research practice 2012, 13, 471-483.
The following ideas first consider what is actually happening when electrolysis is carried out practically.
The idea about what is happening in solution is explored by utilising a role play within class.
This is intended to make students produce evidence which will be contrary to what they believe may
happen in molten solutions via the practical. The role play allows students to physically see and act out
what is happening within the aqueous solution before the idea of the reactivity series is introduced.
Activities
The following practical, taken from the website shown below, will show that different aqueous solutions
that are electrolysed produce different products which can be tested.
This therefore provides the evidence to show that aqueous solutions provide different products. The
equations and reactions are considered in the resulting role play exercise which will undo any
misconceptions that students may have.
http://www.nuffieldfoundation.org/practical-chemistry/identifying-products-electrolysis
Version 1
GCSE (9–1) Gateway Chemistry A
Topic Exploration Pack
Activity 1: The electrolysis of sodium chloride
When molten sodium chloride is electrolysed, the following reactions happen at the cathode and anode.
Cathode: 2Na+(aq) + 2e-
Anode: 2Cl
2Na(s)
-
Cl2(g) + 2e
What happens when sodium chloride, dissolved in water, is electrolysed?
See the website: http://www.nuffieldfoundation.org/practical-chemistry/colourful-electrolysis
During the electrolysis of aqueous sodium chloride, chlorine is made at the anode and hydrogen, instead
of sodium, is made at the cathode. This is because it is energetically easier to reduce the water. (Please
remember that the sodium ions still move towards the cathode in the solution).
The half equations for each electrode are as follows:
Cathode: 2H2O + 2eAnode: 2Cl-
H2(g) + 2OH-(aq)
Cl2(g) + 2e-
When tested, the solution will turn alkaline which confirms the formation of hydroxide ions as shown in
the half equations. The presence of chlorine gas can be detected by moist litmus paper being ‘bleached’
white.
Students will as the research suggests be confused at this point expecting to see sodium being produced
at the cathode when Hydrogen gas is being produced (this can be tested via the squeaky pop method).
Role Play exercise
1.
Two points can be chosen in the classroom such as two benches, one with a positive sign and one
with a negative sign. This represents the electrodes in the system.
2.
Create several A4 signs, two with Na+, two with Cl-, eight with H, four with O and one with a picture
of a bulb.
3.
Choose two pairs of students, with each pair linking arms.
4.
Within each pair, one student holds the Na+ sign while the other student holds the Cl- sign.
5.
These two pairs show an ionically bonded sodium chloride molecule.
6.
Another student can hold the sign of a bulb, showing that the electricity is flowing by holding it up.
7.
The students should then consider what happens when the bond is broken and the Na+ and the Clions move to the cathode and anode respectively.
Version 1
GCSE (9–1) Gateway Chemistry A
Topic Exploration Pack
Extension
The model is then extended. This time, the same sodium and chloride ions are used but then a number
of groups of three students linking arms holding an H, an O and another H, depicting a molecule of H2O,
are introduced.
This time, as ‘the bulb’ lights, the sodium and the chloride ions move to their electrodes but the water
molecule breaks into a hydrogen ion and a hydroxide ion and they both move to their corresponding
electrodes. The two chloride ions join to form chlorine gas and the hydrogen ions join to form a
hydrogen gas molecule. However, the sodium ions and the hydroxide ions remain as spectator ions.
The presence of the OH- ions shows the reason why the solution becomes alkaline.
Activity 2: Reactivity Series
The previous reaction of sodium chloride in water suggests that some species are easier to remove than
others. This can then be related to the reactivity series that the students are familiar with (Table 2).
Table 2: positive and negative ions
Positive ions
Negative ions
K+
F-
Na+
SO42-
Ca2+
NO3-
Mg2+
Cl-
Al3+
Br-
Zn2+
I-
Fe2+
OH-
Sn2+
Pb2+
H+
Cu2+
Ag+
The explanation of the table slightly differs as the reactivity series of metals considers the relative ease
of removing electrons thus making it more reactive. However, this table considers the opposite i.e. the
difficulty of putting an electron back in so the ease of removal by electrolysis increases down the table.
Version 1
GCSE (9–1) Gateway Chemistry A
Topic Exploration Pack
For example, if a solution contained K+ ions and H+, hydrogen gas would be evolved instead of
potassium metal.
When carrying out an electrolysis reaction, the solvent can become part of the reaction, which is why
hydrogen gas is given off when electrolysing sodium chloride solution.
This explains the reactions that you have carried out in the previous experiments.
Part 4
Introduction
In this part, teachers will consider specific reactions such as the electrolysis of copper (II) sulfate and
sodium chloride.
This builds on the work carried out in the previous lessons explained by the role play exercise that the
students have carried out. The students should now be able to predict what happens with these
reactions, however, this does change depending upon the electrode used.
Objective
To describe electrolysis in terms of the ions present and reactions at the electrodes.
The electrolysis of copper sulfate solution
Please see the website shown for this next experiment:
http://www.nuffieldfoundation.org/practical-chemistry/electrolysis-copperii-sulfate-solution
The above experiment can be carried out using both graphite electrodes and copper electrodes to
consider the way in which we can extract copper to plate something or purify a sample of copper.
However, students should be asked to predict what happens based upon the reactivity table (Table 2)
and why, when a graphite electrode is used, oxygen is evolved and the colour of the solution disappears,
whereas when copper electrodes are used, the mass of one electrode increases and the other one
deceases but the solution does not change colour.
Version 1
GCSE (9–1) Gateway Chemistry A
Topic Exploration Pack
The half equations are as follows:
O2(g) + 4H+(aq) +4e-
Graphite anode: 2H2O(l)
Cu2+(aq) + 2e-
Copper anode: Cu(s)
Graphite or copper cathode: Cu2+(aq) +2e-
Cu(s) (for the graphite anode the stoichiometry
needs to be doubled to make the charges balance)
The conclusion should be that when both electrodes are copper, there is just a transfer of pure copper
from one electrode to the other, with the overall numbers of copper ions remaining the same in solution.
This is also the reason why there is no overall change in colour of solution.
In addition, oxygen is given off as the solution is slightly acidic due to the presence of the SO42- ions. So
another conclusion can be gained from this: if the solution is alkaline, hydrogen gas is given off and if the
solution is acidic, oxygen gas is given off when using aqueous solutions.
Example past paper questions that can be used as an end of topic assessment:
http://www.ocr.org.uk/Images/135991-question-paper-unit-b741-02-chemistry-modules-c1-c2-c3-higher-tier.pdf
- May 2012 Gateway science question 12
http://www.ocr.org.uk/Images/175767-question-paper-unit-b741-02-chemistry-modules-c1-c2-c3-higher-tier.pdf
- May 2013 Gateway science question 6b
http://www.ocr.org.uk/Images/175769-question-paper-unit-b742-02-chemistry-modules-c4-c5-c6-higher-tier.pdf
- May 2013 Gateway Science question 11
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